An important trend in networking is the migration of packet-based technologies from local area networks (LANs) to metropolitan area networks (MANs). In the simplest terms, a MAN is a network that spans a metropolitan area. Generally, a MAN spans a larger geographic area than a LAN, but a smaller geographic area than a wide area network (WAN). The rapidly increasing volume of data traffic in MANs is challenging the capacity limits of existing transport infrastructures based on circuit-oriented technologies such as SONET, SDH, and ATM. Inefficiencies associated with carrying increasing quantities of data traffic over voice-optimized circuit-switched networks makes it difficult to provision new services and increases the cost of building additional capacity beyond the limits of most carriers' capital expense budgets. Packet based transport technology is considered by many to be one of the best alternatives for scaling metropolitan networks to meet the demand.
One leading packet based transport technology is Ethernet. Various different standard Ethernet interfaces operate at 10 Mbps, 100 Mbps, and 1 Gbps, thus providing scalability of the service interface. Moreover, as nearly all Internet data packets begin and end as Ethernet frames, carrying data in a consistent packet format from start to finish throughout the entire transport path can eliminate the need for additional layers of protocol and synchronization that result in extra costs and complexities. In addition to efficient handling of IP packets, Ethernet has the advantages of familiarity, simplicity, and low cost.
Although Ethernet is well suited for point-to-point and mesh network topologies, it can be difficult to deploy Ethernet in ring configurations and as a shared media. Rings network configurations act as a shared media and typically use media access control (MAC) mechanisms to manage access across multiple users. Ethernet, in contrast, has evolved to support full duplex switched infrastructures and lacks this type of MAC mechanism. However, much of the existing optical fiber network infrastructure in metro areas is in ring form, largely because incumbent transport technologies, e.g., SONET, are typically deployed over fiber rings.
Ring topologies also enable SONET to implement a fast (sub 50 ms) protection mechanism that can restore connectivity using an alternate path around the ring in case of fiber cuts or equipment failure. Unlike SONET, Ethernet does not have a built-in fast protection mechanism. There are, therefore, great benefits in new technologies that can fully exploit fiber rings (in particular, ring resiliency) while retaining all the inherent advantages of a packet-based transport mechanism like Ethernet.
A number of emerging technologies target metro data transport applications. Among these are the Dynamic Packet Transport/Spatial Reuse Protocol (DPT/SRP) and the IEEE 802.17 Resilient Packet Ring (RPR) standard currently under development. Dynamic Packet Transport is a resilient packet ring technology designed to deliver scalable Internet service, reliable IP-aware optical transport, and simplified network operations. Principally for metropolitan area applications, DPT-based solutions allow service providers to cost effectively scale and distribute their Internet and IP services across a reliable optical packet ring infrastructure. DPT is based on SRP, which is a MAC-layer protocol developed by Cisco Systems for ring-based packet internetworking.
The IEEE 802.17 Resilient Packet Ring (RPR) standard, which may include aspects of both DPT and SRP, offers several important features that have heretofore been exclusive to SONET: efficient support for ring topology and fast recovery from fiber cuts and link failures. RPR technology is expected to provide data efficiency, simplicity, and cost advantages that are typical to Ethernet. In addition, RPR technology solves problems such as fairness and congestion control that have not been addressed by incumbent technologies.
As outlined by the current draft IEEE 802.17 Resilient Packet Ring (RPR) standard (the “standard”), the RPR layer model can be described in terms of the open systems interconnect (OSI) reference model familiar to those having ordinary skill in the art. A simplified block diagram showing the ring and station structure of an RPR implementation is shown in FIG. 1.
Medium access control (MAC) control sublayer, MAC datapath sublayer, and reconciliation sublayers are specified within standard, as are the MAC service interface, and PHY service interface supported by the sublayers. The MAC service interface provides service primitives used by MAC clients to transfer data with one or more peer clients on an RPR ring, or to transfer local control information between the MAC and MAC client. The MAC control sublayer controls the datapath sublayer, maintains the MAC state and coordination with the MAC control sublayer of other RPR MACs, and transfer of data between the MAC and its client. The MAC datapath layer provides data transfer functions for each ringlet. The PHY service interface is used by the MAC to transmit and receive frames on the physical media. Distinct reconciliation sublayers specify mapping between specific PHYs and the medium independent interface (MII).
Resilient packet ring system 100 includes a number of ring stations (station 0 130, station 1 140, station 2 150, . . . and station N 160) interconnected by a ring structure utilizing unidirectional, counter-rotating ringlets. Each ringlet is made up of links between stations with data flow in the same direction. The ringlets are identified as ringlet0 110 and ringlet1 120. This standard allows a data frame to be transmitted on either of the two connected ringlets. For example, a unicast frame is inserted by a source station and copied by the destination station. For efficiency, the destination also strips the now irrelevant stale frame. The portion of a ring bounded by adjacent stations is called a span, and thus a span is composed of unidirectional links transmitting in opposite directions. The RPR dual-ring topology ensures that an alternate path between source station and destination station(s) is available following the failure of a single span or station. Fault response methods include pass-through and protection, as described in the standard.
In order to manage traffic and bandwidth on the ring, one or more fairness algorithms are implemented for data traffic designated as fairness eligible. In general, a station is not permitted to use more than its fair share of available capacity for the insertion of fairness eligible traffic when congestion has been detected on a ringlet. This restriction prevents a station from utilizing a disproportionate share of available capacity by virtue of its relative position on the ring. However, the algorithms specified assume that a client associated with a particular station, e.g., MAC client 170, can accept data at ring rate. If this is not the case, packets may get dropped before they get to packet processor 175 and/or the main buffers of the client where more intelligent dropping algorithms, e.g., the random early drop (RED) algorithm, can be used. One solution is to make intermediate buffers, such as burst buffer 177, very large so that packets are never dropped. However, the cost of adding sufficient memory to support such a solution makes it a less desirable solution.
Accordingly, it is desirable to have mechanisms by which data flow to ring station MAC clients can be controlled. Moreover, it is desirable that such mechanisms operate, to the extent possible, within existing and emerging ring transmission schemes.